Kepler Finds 1235 Planets in Four Months: William Borucki Q&A

NASA announced today that the Kepler space telescope has spotted some 1200 planets in a four-month period of observation, including 68 Earth-size planets. We talked to William Borucki, the principle investigator of the mission, about Kepler's successes, whether there is life on other planets and what the future holds for the telescope.

The Kepler telescope found a massive number of planets from May to September in 2009. What's the breakdown?

William Borucki: We released data on 155,000 stars, including 1235 candidate planets. Of those, 68 are Earth-size; 288 super-Earth-size, which means they're almost twice the size of Earth; 293 are Neptune-size, about four times the size of Earth; and 165 candidates are the size of Jupiter, 11 times the size of Earth. We now have over 1200 candidatesmore than twice the number of planets yet discovered by humansfound in our first four months of collecting data. We'll be putting out a paper in the next day or two to let people see what we found.

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Were there results you did not expect? What surprised you?

One of the biggest surprises is that we're finding 170 stars that have multiple planetary candidate systems. That's an enormous number that blows everything else away. You're seeing systems of planets, like the solar system, and we're learning so much. It's a big, important discovery.

The thing that's special is that we've found 68 Earth-size candidate planets, and 54 candidates in the habitable zone of their stars. Of those in the habitable zone, one of these is a little smaller than Earth; four are a little bit bigger, up to twice the size of Earth; and the other 49 are quite a bit bigger. Some of them are the size of Jupiter, which means their moons could be as large as the Earth. All of the moons orbiting Jupiter are in the habitable zone, because Jupiter is in the habitable zone. You can imagine, among these moons, several have atmospheres.

When you do find an Earth-size planet in the habitable zone, how do you determine the chances that it has life?

We have no way of knowing, and I don't want to guess. We don't know if these planets have atmospheres or not, but I can tell you that we've worked very hard to prove that these candidates aren't planets and we've failed. We need a bit more evidence before we can announce them, but that's what we'll be doing in the coming years. We'll have never-before-seen candidates in or near the habitable zone, but we'll be also be spending a lot of our effort to confirm whether the candidates we know about today are planets.

Finding meaning in so much data, over such a vast amount of space and over a long period of time, must have its difficulties. How did you review the data?

One of the things that we spent a lot of time on was looking at the data and seeing if it was a false positive. For example, if you see two eclipses, then it's a small star. There shouldn't be a big eclipse and a small eclipse if it's a planet. There should be just the transit.

If the planet is hot enough, the life of the planet is cut off when it goes behind the star, so we look for that to see if the shape of the transit is right. The shape of a star passing over another star looks different from a planet crossing over a star. Another thing that bothers us quite a bit is that when you look into the galaxy as we do, there are a lot of little starsbillions of little starsand some of them are close to your targets. So a little star that's going through eclipses, an eclipsing binary, makes you think that it's your star, but instead, it's this background star.

When Kepler operates, it has a blurry image, so it doesn't saturate these stars and can't tell the difference. We have to go through a great deal of effort to make sure what we're finding isn't one of these background stars. We take the Kepler data and, using programs we built that search for what happens during a transit, we can tell if a background star is actually doing the transiting, because its brightness is changing.

We also go to the really big telescopes, and we get very sharp images to see if there's anything around that star that could explain it. When we're through with all those tests, we go to the Keck or the HET or another large telescope and we measure the spectra. If the spectra show that the star is wobbling, we get the mass of a planet. In particular, we can go to the Keck, get the period of the planet and get the epoch (where the planet is on its orbit). And that should exactly match what we have with Kepler. If they have the same orbital period and they occur at the same time, we can say yes, we have a confirmation and can announce it.

How did you develop the general concept of the Kepler Mission?

I did photometry for the Apollo program, and I realized 25 years ago that this would be another way to find planets. I realized that it would be a good method to find small planets. Radio of velocity doesn't do a good job finding small planets with long orbits, but photometry does fine. The trick was to develop a detector system that could have the precision that was required. So I started building photometers, for at least a decade, of different kinds and trying them out at observatories to see how they worked. I was finally convinced that we could build this into a spacecraft and go search for planets. At the same time, NASA came up with a new program called the Discovery Program, and I knew that would be big enough to carry the telescope we needed to put it into space. It took a pretty big rocket to put the Kepler into orbitit was the size of a VW van.

You've said that you have dreamed of discovering life on other planets since you were a kid. Was there one moment where you thought to yourself, wow, this is amazingthis is really going to mean something?

I'm a little hesitant to describe it, but I'll tell you. When I was a kid, I built rockets. And I had the privilege of going up to the big rocket booster, which was 15 stories high, and I thought, this is going to carry my experiment into space. It really made it real to see that giant rocket there, ready to make this work. It was really a special experience.

And of course, the experience of seeing Kepler launch into space, see it arch through the sky and disappear into orbit, that was wonderful, as was listening to the radio signals when it was going to eject its cover. The telescope is launched into orbit with a cover across the lens. And everybody's sitting there in the control room two weeks later, saying, we've checked all the electronics and everything's ready. Let's see if the cover will come off. Sometimes those mechanical things stick, and that's the end of the mission. It was a thrilling time. The cover came off and we got first lightwe saw the stars through it. Wonderful.

What do you plan to do with the Kepler mission next? What do you hope to find?

What the mission is designed for, and I'm going to be a little facetious here, is to be the most boring mission ever launched. It simply takes images every 6 seconds of the skythe same part of the sky, the same group of stars, over and over again. And that is so wonderful. We get this marvelous data set, and the longer you look, the smaller the planets you find. The longer you look, the more planets you find in the habitable zone. This is just the most marvelous mission that has ever been launched. Hubble Space Telescope is a small candle compared to the light of this mission. Year after year, we'll be finding more planets, more planetary systems. And we'll find out the answer to the question "Are earths common?" Are earths in the habitable zone common? And if the answer is yes, life is probably common, too. And that means that we can move forward to new missions that go and explore those planets and find life on those planets.